Add callbacks for EVAL_BEFORE_ALL and EVAL_AFTER_ALL

[rishab-partha]

What does this PR do?

For multiple use cases, such as weight norm calculations, and logging after all evaluators run, we want to add events before and after all evaluators, termed EVAL_BEFORE_ALL and EVAL_AFTER_ALL. To do this, we modify the documentation and the workflow for the _run_evaluators function in order to first cache the results of whether evaluators run and add these events if that occurs.

What issue(s) does this change relate to?

N/A

Before submitting

• Have you read the contributor guidelines?
• Is this change a documentation change or typo fix? If so, skip the rest of this checklist.
• Was this change discussed/approved in a GitHub issue first? It is much more likely to be merged if so.
• Did you update any related docs and document your change?
• Did you update any related tests and add any new tests related to your change? (see testing)
• Did you run the tests locally to make sure they pass?
• Did you run pre-commit on your change? (see the pre-commit section of prerequisites)

[mvpatel2000]

LGTM! Will approve after tests are added to test_event.py

[rishab-partha]

LGTM! Will approve after tests are added to test_event.py

Just added tests!

[mvpatel2000]

LGTM! Thanks!!